In photolithography, a light-opaque pattern imprinted on a mask or reticle is interposed between a radiation source and a photosensitive resist (photoresist) layer on a semiconductor wafer. Typically, the radiation source is a mercury lamp. If the photoresist polarity is positive, the unshaded or exposed portions of the photoresist are easily dissolved or otherwise removed in a subsequent development step. The unexposed portions of the positive photoresist remain polymerized and will not be removed during the development step.
If a negative photoresist is used, the exposed photoresist becomes polymerized and, hence, resistant to developer solutions, while the unexposed negative photoresist is easily dissolved by the developer solution.
After the photoresist is dissolved and removed, the resulting wafer uses the remaining patterned photoresist layer as a protective layer to, for example, block the deposition of dopants or to prevent etching of one or more layers underlying the remaining photoresist.
One type of projection photolithographic process uses a mask (containing the entire wafer pattern) which is spaced close to (e.g., 5 microns) the wafer. In this process, no lens system is required to focus the mask image onto the water surface.
Another type of projection photolithographic process uses a mask spaced away from the wafer, wherein a lens system interposed between the mask and wafer is used to focus the pattern of the mask onto the entire wafer.
An improved type of projection photolithographic process uses a reticle, which contains a pattern for a single die or a relatively small portion of the wafer. This process uses a stepper, wherein the reticle is mounted typically 50 centimeters to 1 meter from the wafer, and a lens system focuses the reticle pattern on a small portion of the wafer to expose the photoresist. The wafer is then slightly shifted relative to the reticle image, and the exposure process is repeated until substantially the entire wafer has been exposed by the same reticle in a repeated pattern.
As is well known, a major limiting factor in image resolution when using any of the above-described projection photolithographic processes is the diffraction of light, wherein light is bent around the mask or reticle pattern. Due to diffraction, the mask or reticle pattern is slightly distorted when the pattern image is projected onto the wafer surface.
Applicants have discovered that, with conventional projection photolithographic methods using conventional masks and reticles, line widths of lines within a dense pattern of lines formed on a wafer surface are narrower than line widths of isolated lines, even though all line widths on the mask or reticle are identical. Such is the case where a positive photoresist is used and the opaque portions of the mask or reticle correspond to the lines and other features to be formed on the wafer surface. Where a negative photoresist is used, causing the clear portions of the mask or reticle to correspond to the lines and other features formed on the water surface, the effect would be opposite.
Thus, the resulting wafer contains feature sizes that are dependent upon whether a feature is isolated or within a dense pattern. This results in unpredicted feature sizes. One skilled in the art of integrated circuit design will be aware of the various problems which may result from unpredicted feature sizes, such as differing electrical characteristics between the formed conductive lines where the lines were designed to have identical electrical characteristics.
The reason for this discrepancy in line widths of lines within a dense pattern and isolated lines is illustrated in FIGS. 1-3. It is to be noted that this discrepancy exists with all geometric shapes and not just lines.
FIGS. 1 and 2 illustrate a simple metalization process.
Initially, wafer 10, shown in FIG. 1b, has formed over its surface an unpatterned layer of silicon dioxide 12. Metal layer 14, typically aluminum, is then deposited on the surface of wafer 10 using conventional techniques. A layer of positive photoresist is then spun onto the surface of the wafer to completely coat the surface of the wafer. Using well-known techniques, the wafer surface is then selectively exposed to radiation through a reticle.
FIG. 1a shows a reticle pattern represented by light blocking portions 16 and 18, which block light generated by a well-known type of lamp used to expose photoresist. Downward arrows represent partially coherent radiation 20 from the lamp. Lens 22 focuses the image of the reticle onto the surface of wafer 10. The X axis of the graph in FIG. 1a represents the distance along the wafer 10 surface, and the Y axis of the graph represents the resulting light intensity on the wafer 10 surface.
As seen by the intensity of light impinging upon the wafer surface, a certain low level of light intensity exists under light blocking portions 16 and 18 due to the diffraction of light, whereby the light waves traveling in straight paths bend around light blocking portions 16 and 18. Thus, additional area of the photoresist is exposed to light due to the diffraction of light. Further, it is seen at the edges of the shaded areas on the surface of wafer 10 that the light waves from radiation 20 have constructively and destructively interfered with one another as a result of the diffraction of light. Hence, where the light intensity is increased due to constructive interference, the photoresist will be even more exposed. The extent of these edge effects is a function of light coherency, numerical aperture of the lens used, and other factors, as discussed in the publications: "Optical Projection Printing," by J. D. Cuthbert, Solid State Technology, Aug. 71; and, "Optical Imaging for Microfabrication," by J. H. Bruning, Journal of Vacuum Science Technology, 17(5), Sept./Oct. 80; both incorporated herein by reference.
It is assumed for purposes of illustration that any photoresist exposed to light above a threshold intensity level L.sub.TH will be dissolved away during development of the photoresist. In actuality, this distinction between exposed and unexposed photoresist is not as precisely defined. This threshold light intensity level is represented on the Y axis of FIG. 1a by L.sub.TH.
FIG. 1b shows wafer 10 after being sufficiently exposed to the light pattern and after the exposed photoresist has been removed. Photoresist portions 24 remain. In this example, the width of photoresist portions 24 is 0.74 microns.
Shown in FIG. 1c, exposed metal layer 14 is anisotropically etched, using well-known techniques, and photoresist portions 24 are removed with a photoresist stripper. The remaining oxide 12 may then be removed if desired.
Thus, what remains is a metal pattern shown in FIG. 1c, comprising parallel metal lines 26 and 28, whose geometries are dictated by the geometries of light blocking portions 16 and 18 and by the spaces between the light blocking portions. The width of light blocking portions 16 and 18 corresponds to metal line widths of 0.74 microns in FIG. 1c. In the example of FIG. 1c, the pitch or distance between the centers of metal lines 26 and 28 is three microns.
In the example of FIG. 1, the pitch of three microns for parallel metal lines 26 and 28 results in an isolated pattern of lines, since the diffraction effect from light blocking portion 16 does not influence the shape of metal line 28, and the diffraction effect from light blocking 18 does not influence the shape of metal line 26.
FIG. 2b shows a dense pattern of metal lines, wherein the pitch between metal lines 30, 31, and 32 is 1.5 microns. As will be seen in the example of FIG. 2, the resulting widths of metal lines 30, 31, and 32 are less than 0.74 microns, even though the widths of light blocking portions 40-42 of the reticle shown in FIG. 2a are identical to the widths of light blocking portions 16 and 18 in FIG. 1a. This is because light blocking portions 40-42 are situated sufficiently close to one another such that the diffraction effects from light blocking portions 40 and 42 cause a greater area of photoresist under center light blocking portion 41 to be exposed above the threshold intensity L.sub.TH. Also, the diffraction effects from light blocking portion 41 cause a greater area of photoresist under light blocking portions 40 and 42 to be exposed above L.sub.TH.
As seen, metal line 31 is narrower than metal lines 30 and 32, since the line width of metal line 31 is reduced on both sides by the diffraction effects of light blocking portions 40 and 42.
The above-described process is merely illustrative of the effect of the diffraction of light inherent in a projection-type photolithographic process. The drawbacks of prior art photolithographic processes stemming from the diffraction effect are in no way limited to the above-described specific process used to form a metalization pattern.
Shown in FIG. 3a is a top view of a simple reticle pattern, wherein rectangles 50, 51, and 52 define opaque portions of the reticle, and portion 56 defines a clear portion. The pattern (highly magnified) transferred to the surface of a wafer is shown in FIG. 3b, where metal lines 50a, 51a, and 52a result from opaque portions 50, 51, and 52, respectively. For purposes of this example, the close proximity of opaque portions 50, 51, and 52 cause lines 50a, 51a, and 52a to be slightly narrowed, similar to that effect illustrated in FIGS. 2a and 2b. For this reason, the resulting pattern of lines 50a, 51a, and 52a is classified as a dense pattern of lines.
Also shown in FIG. 3a is opaque portion 58, which produces metal line 58a in FIG. 3b. Since, there is no influence from adjacent opaque portions, there is no narrowing effect on line 58a. For this reason, line 58a is classified as an isolated line.
Shown in FIG. 4 is a graph illustrating the difference in width between an isolated line (LW.sub.ISO) of 0.7 microns and a line in a dense pattern of lines (LW.sub.DENSE) for a range of pitches, wherein the isolated line and the line in the dense pattern have identical line width geometries on the reticle. This difference in line width stems from what is termed the proximity effect. As seen from FIG. 4, this proximity effect becomes significantly pronounced for pitches less than two microns for line widths of approximately 0.7 microns. For example, at a pitch of 1.5 microns, the isolated line width is 0.7 microns, while the line width in a dense pattern is approximately 0.67 microns. As the state of the art in this field progresses, line widths and pitches will be continually reduced, further compounding the disadvantageous effects of isolated lines being formed wider than lines in a dense pattern.
Thus, what is needed in the field of photolithography is a method to correct for this proximity effect so that a mask or reticle produces the intended pattern on the wafer.